Method for preparing an optical fibre, optical fibre and use of such

ABSTRACT

Methods for coating an optical fiber with optical fiber Bragg grating (FBG) with a hermetic coating, particularly a coating of carbon, are employed to avoid ingress of gases, vapors or fluids in the ambient environment. This ingress can be from water or hydrogen, which can diffuse in the fiber glass and cause deviation/drift in the measured Bragg measurements. Bragg gratings that maintain the grating strength at temperatures in excess of 1000° C. are used and can be formed by heating the fiber above 1000° C. in a chamber with a reactive gas that produces deposition of carbon.

The invention relates to a method for coating an optical fibre as statedin the introductory part of claim 1, as well as an optical fibremanufactured by this method, and a use of such an optical fibre.

BACKGROUND

A fibre Bragg grating (FBG) is a permanent periodic refractive indexmodulation in the core of a single-mode optical silica glass fibre overa length of typically 1–100 mm. It can be created in a photosensitivefibre by transversely illuminating the fibre with a periodicinterference pattern generated by ultra-violet (UV) laser light. Therefractive index modulation in a standard FBG is believed to form by UVinduced breaking of electronic bonds in Ge-based defects, releasingelectrons which are re-trapped at other sites in the glass matrix. Thiscauses a change in the absorption spectrum and in the density, andthereby a change in the refractive index of the glass. An FBG reflectslight within a narrow bandwidth (typically 0.1–0.3 nm), centred at theBragg wavelength, 1B=2 neffL, where neff is the effective refractiveindex seen by the light propagating in the fibre, and L is the physicalperiod of the refractive index modulation. It is known that thereflected Bragg wavelength from an FBG will change with any externalperturbation which changes the effective refractive index seen by thepropagating light and/or the physical grating period (fibre length),such as temperature and strain. By measuring the reflected Braggwavelength, using for example a broadband light source and aspectrometer, an FBG can be used as a sensor for measuring such externalperturbations. A standard UV induced FBG can be made thermally stable upto ca. 300–400° C., at higher temperatures the UV-induced refractiveindex modulation will decay fast and the grating will be erased.

It is possible to make so-called chemical FBGs which can survivetemperatures up to 1100–1200° C. [Fokine, M., Sahlgren, B. E., andStubbe, R., “High temperature resistant Bragg gratings fabricated insilica optical fibres,” ACOFT-96, post-deadline-paper, 1996, Sidney,Australia and PCT patent application WO 98/12586 to Fokine]. A chemicalgrating is typically formed by first writing a standard grating inhydrogen loaded, fluorine (F) co-doped, Ge-doped silica fibres. UVexposure of such a fibre creates OH in the illuminated regions of thefibre which through heating reacts with F to form HF. Post-annealing attemperatures >1000° C. causes the HF to diffuse out of the fibre core,leaving UV-exposed areas more depleted of F than unexposed areas,producing a spatially varying F-concentration and hence a refractiveindex variation (grating). It is also possible to make other types ofspecial FBGs, which can survive high temperatures, such as type II FBGs[W. X. Xie et.al., Opt. Commun. 1993, Vol. 104, pp. 185–195]. It isknown that type II FBGs in germanium-free nitrogen-doped silica-corefibres are much more stable at elevated temperatures than standard typeI FBGs [E. M. Dianov et.al., Electron. Lett., Vol. 33, pp. 236–237,1997].

Several FBGs can be wavelength multiplexed along one fibre, making themvery attractive for distributed measurements of strain and temperature.FBGs can also be used as a pressure sensor by measuring the shift inBragg wavelength caused by hydrostatic pressure induced compression ofthe silica glass fibre [Xu, M. G., Reekie, L., Chow, Y. T., and Dakin,J. P., “Optical in-fibre grating high pressure sensor, Electron. Lett.,Vol. 29, pp. 398–399, 1993]. This provides a very simple sensor designwith small dimensions and good reproducibility and long-term stabilityprovided by the all-silica construction of the sensor. An all-fibre FBGsensor with enhanced pressure sensitivity and inherent temperaturecompensation can be made by using a passive or an active (fibre laser)FBG written in a birefringent side-hole fibre, which has two openchannels symmetrically positions at each side of the fibre core, [Udd,E., U.S. Pat. Nos. 5,828,059 and 5,841,131, Kringlebotn, J. T.,Norwegian patent application 19976012 (passive FBG sensors) andKringlebotn, J. T., U.S. Pat. No. 5,844,927 (active FBG sensor)]. It isalso possible to make FBG pressure sensors with enhanced pressuresensitivity by using a glass transducer element surrounding the opticalfibre, either to convert pressure to strain/compression in the fibre orto convert pressure to fibre birefringence [Udd, E., U.S. Pat. No.5,841,131].

When fibre-optic sensors are operated under conditions of hightemperature, such as in oil wells, there might be considerable drifteffects both in FBG and birefringent interferometric sensors, as taughtus by J. R. Clowes et.al. in “Effects of high temperature and pressureon silica optical fibre sensors,” IEEE Photon. Technol. Lett., Vol. 10,pp. 403–405, 1998. The drift effect occurs when the fibre is surroundedby a liquid, such as water or oil, and increases with increasingtemperature. The effect is believed to be due to ingress of liquidmolecules into the outer layers of the fibre cladding resulting in thedevelopment of a highly stressed layer and consequently a tensile stresson the fibre core. This increases the optical path length of a fibre andchanges the Bragg wavelength of an FBG. This effect will also change thebirefringence of a highly-birefringent fibre. Clowes et.al. demonstratedthat the increase in optical path length of a fibre was reduced by anorder of magnitude using a hermetic, carbon coated fibre.

In addition, diffusion of gases, such as hydrogen, into the fibre, willcause a change in the refractive index proportional to the hydrogenconcentration, and consequently drift in Bragg wavelength of an FBGwritten into the core of the fibre, as disclosed by Malo et.al. in“Effective index drift from molecular hydrogen diffusion inhydrogen-loaded optical fibres and its effect on Bragg gratingfabrication,” Electronics Letters, Vol. 30, pp. 442–444, 1994. Hydrogenwill also cause a loss increase in an optical fibre, which could bedetrimental for FBG-based rare-earth doped fibre lasers. Finally,diffusion of gases into the holes of a side-hole fibre will change thepressure inside the holes, and hence the pressure difference whichaffects the measurement of the external hydrostatic pressure.

As disclosed by Kringlebotn in Norwegian patent application 19976012 apractical all-fibre FBG pressure sensor without drift athigh-temperature operation can be realised by recoating an FBG in aside-hole fibre with a hermetic coating to prevent penetration of gases,vapours or liquids from the surrounding environment. However, there isno mentioning of how such a coating can be applied on an FBG.

A. Hay [U.S. Pat. No. 5,925,879] discloses the use of a carbon coating,or another hermetic coating on an FBG sensor to protect the opticalfibre and sensors from a harsh environment.

Carbon has been shown to provide a good hermetic coating for opticalfibres, making them essentially impermeable to both water and hydrogen,maintaining the mechanical strength and low loss of the fibre. A carboncoating can be applied to an optical fibre during the drawing processbefore the fibre glass cools through a pyrolytic process [see forexample U.S. Pat. No. 5,000,541 to DiMarcello et.al.]. Carbon coatingusing a similar technique can also be applied to splices betweenhermetic fibres to maintain hermeticity after splicing of carbon coatedfibres [U.S. Pat. No. 4,727,237 to Schantz, C. A., et.al.]. In thelatter patent a pyrolytic technique is used based on heating the fibresplice region with a CO2-laser inside a chamber containing a reactantgas causing a carbon coating to form on the glass surface by pyrolysisof the reactant gas. The temperature in the fibre will during such aprocess typically exceed 1000° C. This high temperature pyrolyticprocess have been shown to provide highly hermetic coatings, and seemsto be the preferred technique for carbon coating of optical fibres.However, a standard FBG, i.e. a so-called type-I FBG in agermanium-doped silica fibre cannot be carbon coated using such aprocess, since it will be erased at the high temperature involved.

OBJECTS

The main object of the invention is to provide a method for recoating anFBG in a fused silica optical fibre, or an FBG embedded in a fusedsilica glass element, with a hermetic carbon coating to preventin-diffusion of molecules from the surrounding liquid or gas into theglass at elevated temperatures, hence eliminating or reducing drift inthe Bragg wavelength of the FBG. This is of particular importance forFBG based temperature, strain and pressure sensors operated at elevatedtemperatures, for example in oil-wells, in refineries or in industrialprocessing applications. It is an object to provide a practicalall-fibre pressure and temperature sensor for use in such applications.It is further an object to provide a method that prevents loss in FBGs,resulting from indiffusion of hydrogen. Finally it is an objective thatthe method maintains the mechanical strength of the fibre or glasselement and maintains the grating strength (grating reflectivity).

THE INVENTION

The object of the invention is achieved with a method having features asstated in the characterising part of claim 1. Further features arestated in the dependent claims. The method consists of using a hightemperature carbon coating technique on a special fibre Bragg gratingswhich can survive and maintain their grating strength (reflectivity) atelevated temperatures, typically in excess of 1000° C. These gratingscan be so-called chemical gratings, i.e. a gratings consisting of avariation in the chemical composition along the grating. A The chemicalgrating can be created in a suitable hydrogen loaded optical fibre,typically a germanium, fluorine co-doped silica fibre, where the fibrecan be a low-birefringent fibre or a highly birefringent fibre such as aside-hole fibre. The grating can also be other types of special hightemperature gratings, such as type II gratings, for example ingermanium-free nitrogen-doped silica fibres. The high temperature carboncoating technique will typically be a pyrolytic technique based onheating the fibre in a chamber containing a reactant gas causing acarbon coating to form on the glass surface by pyrolysis of the reactantgas.

The carbon coated FBG should be coated with a second protective coatingwhich protects the carbon mechanically and prevents the carbon fromburning off at elevated temperatures. This secondary coating can be apolyimide, silicone or acrylate coating, or a thin metal coating such asgold.

The carbon coating process can be combined with the high temperatureannealing process needed to create the chemical gratings so that heatingthe fibre inside a chamber containing a reactant gas both creates thechemical grating and causes a deposition of a carbon coating on theglass surface. This reduces the number of processing steps and minimisesfibre handling, providing a high mechanical strength of the grating. Thesecondary recoating process can also be incorporated inside the chamberby putting the carbon coated fibre in a mould filled with the coatingmaterial and heating this with an internal heater to cure the coatingmaterial and form a suitable coating.

A particular use of an optical fibre according to the invention isstated in claim 12. Further, a particular embodiment of an optical fibreaccording to the invention is stated in claim 13.

EXAMPLES

In the following, the invention will be described with reference toillustrations, where

FIG. 1 shows the cross-section of a chemical FBG written in a side-holefibre with a carbon coating and a secondary polyimide coating.

FIG. 2 is a schematic illustration of a chamber for recoating a hightemperature FBG with carbon.

FIG. 3 shows the long-term drift in Bragg wavelength of a chemical FBGexposed to silicon oil at 200° C., both with and without carbonrecoating.

FIG. 1 shows the cross-section of a chemical FBG in the core (11) of aside-hole fibre (12) with a carbon coating (13) formed by a pyrolyticprocess and a secondary polyimide coating (14) to protect the carboncoated FBG.

FIG. 2 is a schematic illustration of a chamber for recoating a hightemperature FBG, such as a grating, with carbon, A chemical hightemperature grating (21) in a stripped section (22) of an optical fibre(23) is placed inside a sealed chamber (24). The fibre is entering andexiting the chamber through pressure seals/penetrators (25). A gasmixture (26) of a reactive gas, for example acetylene, and nitrogen isentering the chamber through an inlet (27) and leaving through an outlet(28), creating a gas flow with a slight over-pressure inside the chamberkeeping other gases out. The fibre is heated by a scanning CO₂-laserbeam (29) going into the chamber through a transmitting window (30).

FIG. 3 shows the long-term drift in Bragg wavelength of a chemical FBGexposed to silicone oil at 200° C., with (1) and without (2) a carboncoating, showing that a carbon coating eliminates the wavelength driftin such an environment, believed to be due to stress changes in thefibre caused by ingress of molecules from the surrounding oil into theouter layers of the fibre cladding.

1. A method of providing a hermetically protected fiber optic Bragggrating (FBG) to limit ingress by diffusion into optical fiber glass offluid from a surrounding environment, comprising: writing a Bragggrating into an optical fiber, wherein the Bragg grating maintains itsstrength at temperatures in excess of 1000° C.; and forming a hermeticcoating on the optical fiber with a process that includes heating of theoptical fiber.
 2. The method according to claim 1, wherein writing theBragg grating comprises: inscribing a precursory Bragg grating into theoptical fiber by illumination with ultraviolet light; and thereafterheating the optical fiber to form the Bragg grating having a periodicchemical variation in composition along the optical fiber with acorresponding periodic variation in refractive index due to componentsof the optical fiber being gassed/diffused out of the optical fiberthereby producing a refractive index variation.
 3. The method accordingto claim 2, wherein a single heating process is used for heating of theoptical fiber while forming the hermetic coating and heating the opticalfiber to form the Bragg grating.
 4. The method according to claim 2,wherein the periodic chemical variation is formed in a hydrogen loaded,fluorine co-doped, germanium doped silica fiber that provides theoptical fiber.
 5. The method according to claim 1, wherein writing theBragg grating and forming the hermetic coating are conductedsimultaneously.
 6. The method according to claim 1, wherein writing theBragg grating includes writing the Bragg grating of type II.
 7. Themethod according to claim 1, wherein writing the Bragg grating includeswriting the Bragg grating of type II into a germanium-freenitrogen-doped silica fiber that provides the optical fiber.
 8. Themethod according to claim 1, wherein forming the hermetic coating is aresult of a pyrolytic process, the pyrolytic process including pyrolysisof a reactive gas causing formation of a carbon coating deposited on theoptical fiber to provide the hermetic coating.
 9. The method accordingto claim 8, further comprising coating the optical fiber with anadditional coating to protect the carbon coating against mechanicaldamage and damage by heating.
 10. The method according to claim 8,further comprising coating the carbon coating disposed on the opticalfiber with an additional coating of polyimide, silicone or acrylate. 11.The method according to claim 8, further comprising coating the carboncoating disposed on the optical fiber with a thin metallic coating. 12.The method according to claim 8, wherein the reactive gas is acetylene.13. The method according to claim 8, further comprising coating thecarbon coating disposed on the optical fiber with a gold coating. 14.The method according to claim 1, wherein heating of the optical fiber isperformed using a laser beam.
 15. An optical sensor providing ahermetically protected fiber optic Bragg grating (FBG) to limit ingressby diffusion into optical fiber glass of fluid from a surroundingenvironment, comprising: an optical fiber having a Bragg grating writtentherein, wherein the Bragg grating maintains its strength attemperatures in excess of 1000° C.; and a pyrolytic hermetic coatingformed on the optical fiber by heating of the optical fiber.
 16. Theoptical sensor of claim 15, wherein the optical fiber is configured toprovide temperature and hydrostatic pressure measurement.
 17. Theoptical sensor of claim 16, wherein the coating is a carbon coating. 18.The optical sensor of claim 15, wherein the optical fiber is configuredto provide hydrostatic pressure measurement.
 19. The optical sensor ofclaim 18, wherein the optical fiber has a core and cladding with theBragg grating written into the core and the cladding including twoside-holes.
 20. The optical sensor of claim 19, wherein the opticalfiber is spliced in between standard single mode fibers, so that achange in pressure difference between the surrounding environment andthe side-holes causes a change in the birefringence of the optical fiberand hence a change in wavelength difference between two reflection peaksof the grating according to each of two orthogonal polarized eigenmodesof the optical fiber.